Our principal objective is the determination of the three-dimensional structures of immunoglobulins from patients with multiple myeloma and/or amyloidosis. We are currently emphasizing the serum IgGl protein and the urinary Bence-Jones dimer from the patient Mcg. A low resolution (6.5 A) electron density map has been calculated for the Mcg IgGl protein. This map is being interpreted by a combination of computer graphics and a computer search program provided by Navia, Silverton, and Davies at N.I.H. The Mcg Bence-Jones dimer crystallizes in a trigonal form in ammonium sulfate and in an orthorhombic form in water. We previously solved the structure of the trigonal form to 2.3-A resolution and built an atomic model. An electron density map has been computed for the orthorhombic form at 6.5 A resolution and interpreted with a computer graphics system. Even at low resolution, it is clear that there are marked structural differences in the Mcg Bence-Jones dimer in the two solvent systems. These results emphasize that immunoglobulin gene products are sufficiently flexible to have multiple conformations.